The burden, causes, and determinants of blindness and vision impairment in Asia: An analysis of the Global Burden of Disease Study

Background Asia accounts for more than half of the world’s population and carries a substantial proportion of the global burden of blindness and vision impairment. Characterising this burden, as well as its causes and determinants, could help with devising targeted interventions for reducing the occurrence of blindness and visual impairment. Methods Using the Global Burden of Disease Study 2019 database, we retrieved data on the number of disability-adjusted life years (DALYs); crude and age-standardised rates; and the prevalence (with 95% uncertainty intervals (95%UIs)) of blindness and vision loss due to six causes (age-related macular degeneration, cataracts, glaucoma, near-vision impairment, refractive error, and other vision loss) for Asian countries for the period between 1990 and 2019. We defined DALYs as the sum of the years lost due to disability and years of life lost, and calculated age-standardised figures for the number of DALYs and prevalence by adjusting for population size and age structure. We then evaluated the time trend of the disease burden and conducted subgroup analyses by gender, age, geographic locations, and socio-demographic index (SDI). Results In 2019, the DALYs and prevalence of blindness and vision loss had risen by 90.1% and 116% compared with 1990, reaching 15.84 million DALYs (95% UI = 15.83, 15.85) and 506.71 million cases (95% UI = 506.68, 506.74). Meanwhile, the age-standardised rate of DALYs decreased from 1990 to 2019. Cataracts, refractive error, and near vision impairment were the three most common causes. South Asia had the heaviest regional disease burden (age-standardised rate of DALYs = 517 per 100 000 population; 95% UI = 512, 521). Moreover, the burden due to cataracts ranked high in most Asian populations. Being a woman; being older; and having a lower national SDI were factors associated with a greater vision loss burden. Conclusions The burden due to vision loss remains high in Asian populations. Cataracts, refractive error, and near vision loss were the primary causes of blindness and vision loss. Greater investment in ocular disease prevention and care by countries with lower socioeconomic status is needed, as well as specific strategies targeting cataract management, women and the elderly.

Vision loss (including blindness and vision impairment) significantly impacts both an individual's quality of life and a society's economic development [1][2][3][4][5][6], making it a significant public health issue.The World Health Organization (WHO) has suggested that at least 2.2 billion people were blind or visually impaired worldwide in 2019, half of whom had conditions that were preventable or treatable [7,8].
Initiatives such as 'VISION 2020: The Right to Sight' headed jointly by the International Agency for the Prevention of Blindness and the WHO, along with 'Towards Universal Eye Health: A Global Action Plan 2014-2019' led by the World Health Assembly, have provided a framework and impetus for member states to eliminate preventable blindness [9][10][11].Yet despite potential modest decreases in the global burden of blindness and vision impairment, existing studies indicate that the specific targets of these plans have not been met in terms of crude prevalence of moderate or worse distance vision [12,13].Therefore, a basic understanding of the current challenge we are facing is crucial for the development and implementation of future health policies.
With an estimated population of 4.61 billion in 2019, Asia accounts for the majority of the world's population (7.74 billion in total), meaning it also carries a substantial proportion of the global burden of blindness and vision impairment [14][15][16][17][18].This burden is expected to increase with the ageing and growing regional population unless further action is taken.In view of these changes, and to assess the impact of global initiatives to restore and protect vision, our knowledge of Asia's regional burden of blindness and vision loss needs to be updated, while shifts in its specific causes and determinants must be better understood.Identifying similarities and disparities between different sub-regions of Asia may assist in health care policy decision-making and allocation of resources.
In recent decades, studies have found that the burden of blindness and vision loss was consistently high in Asia [14][15][16][17][18][19][20][21].One study indicated that South Asia had the highest visual impairment prevalence (23.6%; 95% uncertainty interval (UI) = 19.4-29.4%) in 2010, higher than other regions of the world [22].However, most of these studies either focussed on specific populations in Asia or are currently outdated.A comprehensive and up-to-date evaluation of the region as a whole and with interregional comparisons of blindness and vision loss is needed to inform future strategies.
In this study, we aimed to evaluate the burden of blindness and vision loss using data for the prevalence and disability-adjusted life years (DALYs) in absolute numbers, as well as per-population rates in Asia and its sub-regions between 1990 and 2019.We also investigated various demographic and socioeconomic determinants of vision loss and compared the disease burden among Asian populations with global figures.

Data sources
The Global Burden of Disease (GBD) Collaborator Group uses all available data on disease occurrence, natural history, and severity, providing it meets predefined inclusion criteria.Within the most recent GBD 2019 study, they evaluated the burden of 369 diseases and injuries in 204 countries and territories since 1990, including blindness and vision loss due to six causes (age-related macular degeneration, cataracts, glaucoma, near vision impairment, refractive error, and other vision loss).The methodology of this study has been described elsewhere [23][24][25].In brief, primary data were gathered from various sources, including censuses; household surveys; civil registration and vital statistics; disease registries; health service utilisation records; and others.Using GBD 2019 data (retrieved via the Global Health Data Exchange (GHDx) [26]) for blindness and vision loss, we calculated the disease burden in the terms of DALYs and prevalence for each Asian country and Asia as a whole.To evaluate the disease burden across Asia and assess the variation between different geographical areas, we separated Asia into five geographical sub-regions: Central Asia, East Asia, South Asia, Southeast Asia, and West Asia [27].
We also used the socio-demographic index (SDI), a composite index reflecting a country's socio-demographic development status, to measure the impact of social development on health care services.This comprehensive indicator ranges from 0 to 1, with higher values indicating a lower total fertility rate of women under 25 years of age and higher per capita income and educational attainment (mean education for those aged 15 and older).Per the SDI, countries are classified into five groups: low (SDI < 0.454), lower-middle (SDI = 0.454-0.606),middle (SDI = 0.607-0.689),upper-middle (SDI = 0.690-0.804),and high SDI (SDI ≥ 0.805) [28].

Analytic approach
We defined DALYs as the sum of the years lost due to disability (YLD) and years of life lost (YLL) using the following formula: Number of DALYs = (number of deaths × standard life expectancy at the age of death in years) + (prevalence of condition × disability weight of condition).
We also determined the corresponding uncertainty intervals (UIs), defined as the 2.5% and 97.5% values of the ordered draw [25].We calculated standard life expectancy from the lowest observed risk of death for each fiveyear age group in all populations greater than five million [29].Disability weights were assessed in a previously published study, with the severity of health loss associated with a single given health state [30].These disability weights are measured on a scale from 0 to 1, with 0 implying a state equivalent to full health and 1 a state equivalent to death.Lastly, we calculated age-standardised figures for the number of DALYs and prevalence by adjusting for population size and age structure.
We retrieved the following data for further analyses: the population of Asian countries in 1990 and 2019; the number of DALYs and prevalence of blindness and vision loss, both overall and cause-specific, among Asian countries in 1990 and 2019; the gender-specific, age-standardised number of DALYs and total and cause-specific prevalence of blindness and vision loss among Asian countries and globally in 1990 and 2019; the age-specific number and age-standardised rate of DALYs due to blindness and vision loss (both total and cause-specific) in Asia in 2019.We used the Wilcoxon sign-rank test to compare differences in DALYs between Asia and the global population.
We performed all analyses in Stata/MP, version 15.1 (StataCorp LLC., College Station, Texas, USA) and generated all figures using the GraphPad Prism softwarem version 5.01 (GraphPad Software, San Diego, California, USA).

Overview
The Asian population grew by 40% in 2019 (4.61 billion) compared to 1990 (3.20 billion) and still contributed to a large proportion of the global burden of blindness and vision loss.The prevalence and total DALYs due to blindness and vision loss were 506.71 million (95% UI = 506.68,506.74) and 15.84 million (95% UI = 15.83,15.85), respectively (Table 1), accounting for 71.0% and 70.2% of the global total.After adjusting for the population and age structure, the age-standardised rate of DALYs and prevalence of blindness and vision impairment per 100 000 population were 376 (95% UI = 372, 380) and 10 930 (95%UI = 10 910-10 950), respectively.Compared to 1990, the total disease burden in terms of DALYs and prevalence of blindness and vision impairment increased up to 2019 by 90.1% and 116%, respectively, though the age-standardised rates decreased by 15.9% and 3.35%.

Regional discrepancy
South Asia had the highest burden among all Asian sub-regions.
West Asia had the greatest regional burden of age-related macular degeneration and glaucoma when comparing age-standardised rates.In contrast, Southeast Asia had the highest burden in terms of cataracts, while South Asia ranked first in near vision loss and refractive disorders (Table 1).

Gender discrepancy
We found substantial gender differences in the burden of blindness and vision loss in Asia.The age-standardised rate of DALYs among women was greater in Asia as a whole (men: 349 per 100 000 population; 95% UI = 345, 352 vs women: 392 per 100 000 population; 95% UI = 389, 397) and in all regions except West Asia (Table S1 in the Online Supplementary Document).We also observed this greater burden among women for prevalence in Asia as a whole (men: 10 350 per 100 000 population; 95% UI = 103 330, 10 370 versus women: 11 460 per 100 000 population; 95% UI = 11 440, 11 480).

Socioeconomic difference
We found that the degree of development may impact the burden of blindness and vision loss.Generally speaking, countries with higher SDI had a lower disease burden of blindness, as well as vision loss of all causes between 1990 and 2019 when compared to those with lower SDI (Table 2).

Change of disease burden with age
Regarding the age-stratified disease burden of blindness and vision loss in 2019, the maximum DALY burden occurred in individuals aged 65 to 69 years both in Asia (Figure 2, Panels A and B) and worldwide (Figure 2, Panels C and D).Meanwhile, the age-specific rate of DALYs increased with age, both globally and in Asia.The leading cause of vision loss gradually shifted from refractive error to cataracts after the age of 40 years.The age-standardised burden of DALYs was greater in Asia than globally for cataracts (P = 0.001), near vision loss (P < 0.001), and refractive error (P < 0.001), but lower for glaucoma (P = 0.004).

DISCUSSION
In this study, we gave a comprehensive view of the burden of blindness and vision loss in terms of prevalence and DALYs in Asia between 1990 and 2019.The total burden of vision loss has increased, as Asia's population has grown and aged.However, due to coordinated national efforts, the risk of vision loss for any particular person (represented by age-adjusted rates) has decreased over the last 30 years.We found that, among the five Asian regions, South Asia had the highest burden in terms of DALYs and prevalence after adjusting for population and age structure.This is consistent with a previous study using the GBD database, which also showed that South Asia accounted for one-third of global blindness in 2015 [17]; this study also indicated that a total of 11.76 million people (32.65% of the worldwide total) experienced blindness due to various ocular diseases.
Though we observed interregional discrepancies, the age-adjusted rate of disease burden due to blindness and vision loss in Asia has had a decreasing trend in the past decades.In general, the overall economic improvement among developed and developing countries and regions in Asia, alongside efforts aimed at ensuring increased accessibility to eye care, as well as a sufficient number of facilities and eye care providers, and better health care coverage [30], have led to improved ocular disease outcomes.The age-standardised rate of disease burden were decreasing while we are still facing severe challenges due to the large population of Asia.While the patterns of cause-specific vision loss varied substantially by sub-regions, we observed thrends that were consistent across Asia as a whole.Women, older persons, and those residing in countries with a lower SDI had a heavier disease burdens.
Specifically, we found that women suffered a heavier burden of blindness and vision loss compared to men, which is consistent with previous reports [30][31][32].Women have a greater life expectancy, leading to a higher risk of developing age-related ocular diseases such as cataracts, near vision loss and age-related macular degeneration [33][34][35].Anatomical (as with angle-closure glaucoma) and hormonal (as with cataracts) factors may also play a role [36,37].Moreover, prior work has concluded that inequitable access to eye care services is also an important determinant of the additional burden of blindness and vision impairment among women [12].Our results highlight the need for programmes to improve women's access to eye care services throughout Asia.For example, community outreach vision screening services could help improve equity, particularly for conditions like cataracts and glaucoma, by ensuring improved access to eye care services for women [12].
The leading causes of blindness and vision loss all increase with age.Here we presume that population ageing is a key reason for why the total disease burden for Asia rose, while the age-standardised rate of DALYs and prevalence declined between 1990 and 2019.Recently, the world's population of people aged 65 years or above surpassed the number of those under the age of 5 years for the first time [13,38].Vision loss places a particular burden on the elderly, including an increased risk of falls [39,40] and increased all-cause mor- tality [12,41].Moreover, improved diagnostic surveillance and updated diagnostic tools may explain the change in the disease burden of blindness and vision loss [42,43].Governments in Asia need to invest further in initiatives to increase access to and uptake of eye care services among older persons.Concerning this, health technology assessments are intended to be implemented for evaluating the cost-effectiveness of treating different ocular diseases in the elderly.This is crucial due to constrained resources and the significant challenge of addressing potentially unlimited demands, leading to the necessity of making choices and often rationing services [44].
We also saw that countries with a lower SDI had higher burdens of blindness and vision loss.This presumably reflects a shortage of material and human resources to deliver vision care in low SDI countries, a determinant of blindness which is well-described [45][46][47][48].As noted above, effective and low-cost treatments for the most important causes of vision loss in Asia, including refractive error, cataracts, and near vision loss, could be helpful in this regard.Consequently, we hope that investment in these low-cost treatments across the region will be spurred by increasing evidence of a very high return on investment and significant gains in economic productivity [49][50][51].Moreover, increased economic and education levels were found to enhance awareness and increase the utilisation of eye care services [12].Lastly, improving the availability of cataract and refractive services in resource-limited settings may contribute to the earlier identification and management of early glaucoma and retinal diseases that would have an irreversible impact on visual health.
The most readily treatable causes of blindness and vision loss -cataracts and refraction disorders [10,52] -accounted for the majority of the disease burden among Asian populations, which is consistent with previous studies [14-  [18] and others [60,61] have focused on this problem; in fact, rates for cataract surgeries in some Asian countries such as India have been approaching those of high-income countries.The reason for the high blindness and vision loss burden due to cataracts may lie in the fact that cataract surgery must be performed by a trained surgeon in a facility with the capacity to conduct operations and handle postoperative complications.Consequently, the imbalance of regional development leads to a geographically unequal distribution of surgeries performed by adequate surgeons with proper equipment [62,63].Sending experienced surgeons and surgical equipment to remote areas and providing training programmes for junior ophthalmologists in rural regions to enhance surgical service capacity has now become a trend in developing countries [64][65][66].Regarding refractive errors, China launched a national myopia management programme in 2018, coordinating activities of multiple ministries with the aggressive target of annual reductions of 0.5% in myopia prevalence [67].
Being historically underaddressed, near vision loss is another important cause of blindness and vision loss.The definition used by the GBD [68,69] indicates that this is a reversible condition, readily correctable at low cost and high efficacy with glasses.The fact that the burden of vision impairment and associated disability remains so high, even in Asia's rapidly developing countries, signifies that more must be done to target this problem.The importance of the problem is underscored by high-quality evidence showing the impact of near-vision impairment on workplace productivity [50].Studies have also suggested that lower educational levels and a lack of private health insurance are associated with higher risk of near vision loss [70,71].
One strength of this study is the inclusion of data from across the sub-regions of Asia using a standardised and widely-accepted approach [25].We must also acknowledges ome limitations, most of which are similar to those in the GBD study -most notably the restricted data availability and statistical assumptions [24].Besides this, the disease burden of blindness and vision loss could be underestimated in districts with inadequate ophthalmologic resources to carry out epidemiological research.Possible solutions may be to actively engage with local communities, health care providers, and authorities, and synergistically gather more comprehensive and precise data on the prevalence and impact of ocular diseases.The current digital technologies, telemedicine, and artificial intelligence may have promising roles in evaluating the disease burden in these remote districts.Additionally, future studies could help us better understand the cost-effectiveness evaluation of different strategies and policies for screening and treating ocular diseases.
Nevertheless, our analysis is the first to report the overall disease burden in terms of DALYs and the prevalence of blindness and vision loss among the Asian population.Our findings underscore the urgency for targeted policies and strategies to address the specific challenges faced by Asian regions, especially South Asia, in combating visual impairment.Here we offer specific advice for informing policies and setting pri-orities for action, including the need for initiatives targeting women and the elderly, but also to invest in low-cost care with high rates of return and proven impact on improved outcomes, such as cataract surgery and near and distance refractive services.

CONCLUSIONS
The burden of blindness and vision loss remains high in Asian populations.Preventable blindness, such as cataracts and refractive errors, which contribute to a large proportion of the burden, could be solved at a relatively low cost.We urge policymakers to carefully consider these issues.As subgroups of lower socioeconomic status, women and the elderly bear a significantly heavier disease burden, necessitating focussed prevention for these populations.

Figure 1 .
Figure 1.Contribution of age-related macular degeneration, cataracts, glaucoma, near vision loss, refraction disorders and other vision loss to the overall DALYs in central, east, south, southeast, west and total Asia in 2019.

Figure 2 .
Figure 2. Age-specific DALYs number and rate of blindness and vision loss in 2019 among Asian and global population.Panel A. DALYs number among Asian population.Panel B. Rate of DALYs among Asian population.Panel C. DALYs number among global population.Panel D. Rate of DALYs among global population.

Table 1 .
Total and regional cause-specific disease burden of blindness and vision impairment for Asia from 1990 to 2019 with 95% UIs in parentheses

Table 2 .
Age-standardised rate of DALYs for blindness and vision loss by SDI from 1990 to 2019 with 95% UIs in parentheses